The present invention relates to light splitters as well as optical transmitter configuration with a light splitter.
The present invention is based on problems such as were recognized when employing X-cubes and will be explained in the following. But the findings can, in principle, be transferred to optical light splitters which, as will yet be explained, are used in connection with light of different polarization. Regarding this topic, reference can be made to A. Thelen, xe2x80x9cNonpolarizing interference films inside a glass cubexe2x80x9d, Appl. Optics, Vol. 15, No. 12, December 1976.
For example, in DE 40 33 842 a cuboid optical structural component composed of single prisms with dichroic layers is referred to as a xe2x80x9cdichroic prismxe2x80x9d. WO98/20383 by the sage applicant.
The following definitions are used:
Light,
visible light: light with maximum energy in the spectral range 380 nm-720 nm
red light: light with maximum energy in the spectral range 580 nm-720 nm, in particular in the spectral range 600 nm-680 nm
green light: light with maximum energy in the spectral range 490 nm-605 nm, in particular in the spectral range 500 nm-600 nm
blue light: light with maximum energy in the spectral range 380 nm-510 nm, in particular in the spectral range 420 nm-500 nm
yellow light: light with maximum energy in the spectral range 475 nm-605 nm, in particular at 582xc2x13 nm
white light: light with red, blue and green light components
transparent: negligible absorption in the spectral range 380 nm-720 nm
cube: spatial shape formed by identical rectangles pairwise opposing each other in parallel
First, the fundamental effect of an X-cube will be explained with reference to FIG. 1. Optic light splitters of this type are employed primarily in projectors in order to separate white light into red, green and blue light or to recombine the latter into white light. According to FIG. 1, an X-cube comprises four single prisms 2a-2d. The prisms commonly comprise BK7 glass. In cross section they form right-angled isosceles triangles with an angle of 90xc2x0. The length of the hypotenuse is, for example, between 5 mm and 50 mm, preferably 20-30 mm. Between the two prism pairs 2a and 2b, on the one hand, and 2d and 2c, on the other, a spectrally selectively reflecting and transmitting coating system 5 is embedded, which largely reflects blue light but largely transmits green and red light.
Between the two prism pairs 2a and 2d, on the one hand, and 2b and 2c, on the other hand, a further spectrally selectively reflecting and transmitting coating system 7 is embedded which largely reflects red light, however largely transmits green and blue light.
Consequently, on the X-cube three channels result for red, green and blue light, KR, KG, KB and one channel KR+B+G for white light. On each of the coating systems 5, 7, between the addressed prism pairs, reflection at 45xc2x0 of the incident light, thus colored light, takes place. The hypotenuse faces of the prisms 2 can be coated with an antireflection coating system.
Such X-cubes are mainly used today in projection apparatus in order to recombine red (R), blue (B), and green (G) light, each of which is supplied via light valves, in particular LCD light valves, operating in transmission to the associated channels KR, KB, KG in channel KR+B+G into white light. This is indicated in FIG. 1 in dashed lines. Light valves are therein image-forming elements comprising a multiplicity of individually driven pixels. The number of pixels therein yields the resolution according to EVGA, SGA, EGA, or XGA standards, etc.
Due to the printed conductor and the driving electronics a lower limit of the pixel size exists in the case of such light valves operating in transmission and it is only with difficulty possible to attain sizes below this limit. When decreasing the pixel size, furthermore, the optical aperture per pixel decreases.
The restriction does not apply in light valves which do not work in transmission but rather, as shown in solid lines using light valves LCD in FIG. 1, operate in reflection and therein rotate the plane of polarization of the reflected light by 90xc2x0.
The use of such light valves operating in reflection has been hindered until today by problems which will be explained later. In FIG. 2 the conditions are shown which obtain when replacing conventional light valves, which, according to FIG. 1, are LCD valves operating in transmission, by light valves RLV, which are reflective light valves operating in reflection. If, to the configuration according to FIG. 1, a light valve RLV operating in reflection is connected according to FIG. 2, for example, reflected S-polarized (direction of oscillation of the E field) blue light B reflected on coating system 5 of the X-cube according to FIG. 1, is converted on the light valve RLV into P-polarized blue light and reflected back onto the coating system 5 and, again, reflected by the latter. On one and the same coating system 5, according to FIG. 2, and, analogously, for red light on system 7, reflections of light of identical spectra but different polarizations occur.
Spectrally selectively reflecting and transmitting coating systems, such as are used in said X-cubes but also in other light splitters for color-selective effects, are conventionally produced by means of dielectric multicoating systems. These comprise each at least one layer of a material with lower refractive index and one layer of a material with higher refractive index. For example, as the material with lower refractive index SiO2 is conventionally used, with a refractive index of 1.46. As the material with higher refractive index, for example, TiO2 is used today with a refractive index of 2.4 or Ta2O5 with a refractive index of 2.1.
In FIG. 3 the reflection of S-polarized blue light on a color-selective coating system comprising SiO2/TiO2 is shown as well as that of P-polarized blue light on the same coating system. Both measurements took place at an angle of light incidence of 45xc2x0, as depicted in FIG. 2.
In FIG. 4 is shown on a coating system again composed of SiO2/TiO2 layers and selectively reflecting red light R, the reflection behavior of S-polarized and of P-polarized red light. The measurements of FIGS. 3, 4 were carried out on an X-cube with BK7 glass as the base body material wherein the listed color-selective coating systems 5, 7 or FIG. 2 were embedded.
In FIGS. 3 and 4 is evident that, on the one hand, in both cases the reflection of P-polarized light is significantly less than that of S-polarized light, quite pronounced on the red-selective coating system, and that further a marked edge shiftxe2x80x94polarization shiftxe2x80x94of the reflected spectra takes place. For example, with selective reflection of blue light the 50% reflection points for S- and P-polarization are spaced over 70 nm apart, corresponding to xcex94B.
If in FIG. 2 the represented path of rays is considered, without taking into account the second color-selective coating system 7 provided on the X-cube, thus only the reflection on one coating system, namely the coating system 5 for blue light, one obtains
Ixe2x80x2BoutP=IinSxc2x7RRBPxe2x80x83xe2x80x83(1)
where
IBoutP: intensity of the P-polarized blue light reflected back by coating system 5,
IinS: intensity of the S-polarized blue light incident on coating system 5,
RRBS: the reflection of the blue-selective coating system 5 for S-polarized blue light,
RRBP: the reflection of the blue-selective coating system 5 for P-polarized blue light.
Starting from the reflection behavior shown in FIGS. 3 and 4 for blue light B on coating system 5 according to FIG. 2 and, analogously, for red light R on coating system 7, taking into consideration the particular transmitting coating systems, thus system 7 for blue light B or system 5 for red light R, respectively, intensity spectra for IBoutP or for IRoutP are obtained as shown in FIG. 5 or FIG. 6, respectively. Taking into consideration the stated transmissions, as well as according to FIG. 2 the reflection behavior of a potentially added light valve RLV operating in reflection, for blue light is obtained:
IBoutP=IinSxc2x7TRRSxc2x7RRBSxc2x7RRLVBxc2x7RRBPxc2x7TRRPxe2x80x83xe2x80x83(2)
Therein denote further
TRRS: the transmission of the red-selective coating system 7 for S-polarized light
RRLVB: the reflection of the light valve
TRRP: the transmission of the red-selective coating system 7 for P-polarized light.
The expression for red light is obtained analogously.
The differing reflection properties of the selective red or blue coating systems with respect to S- and P-polarization lead to serious consequences:
The light from the spectral ranges xcex94B or xcex94R between the S- and P-reflection spectra according to FIGS. 3 and 4 is not output but passes correspondingly the blue or red reflector and remains as scattered light in the system, according to FIG. 2 in the X-cube.
In FIG. 7 the spectrum of the scattered light in the X-cube is shown with the stated intermediate spectral ranges xcex94B and xcex94R according to FIGS. 3 and 4 entered.
It is evident that a large quantity of scattered light remains in the system. Further, as evident in FIGS. 5 and 6, the total transmission in the red spectral range as well as also in the blue is insufficient, i.e. substantially less than it would be, in view of FIG. 2, if on both color-selective coating systems 5 and 7 only S-polarized light were to be reflected.
Starting from the described problems, the present invention poses the task of creating light splitters of the above stated type in which the differing color-selective behavior of the coating systems with respect to differently polarized light, is reduced or corrected, or to create an optical transmitter configuration with a light splitter with which this is also attained. As the light splitter of the transmitter configuration according to the invention is preferably used in combination a light splitter according to the invention.
Further, the scattered light in the light splitter or in the transmitter configuration is to be minimized and, if appropriate, the color location is to be optimized.
This task is solved, on the one hand, in the case of a light splitter of the type described in the introduction thereby that for the lower refractive index NLS applies:
1.70xe2x89xa6NLSxe2x89xa62.1.
As will yet be explained, this leads to the fact that the edge shift xcex94xe2x80x94the polarization shiftxe2x80x94of the P- and S-reflection spectra, as explained in conjunction with FIGS. 3 or 4, is substantially reduced and, in addition, the reflection of P-polarized light is approximated to the value for S-polarized light.
In a preferred embodiment, the refractive index NLS is selected as:
NLS=1.8xc2x12%
As the material with higher refractive index is preferably used a material comprising at least as the major component an oxide or oxynitride, therein preferably a material from the series TiO2, Ta2O5, Nb2O5, HfO2, ZrO2, SiOxNy, especially preferred TiO2 and/or Ta2O5. All of these materials have indices of refraction of maximally 2.1.
The lower refractive index NLS selected according to the invention is preferably controlled thereby that as the associated material a mixed material is used comprising at least two materials m1 and m2, for the refractive index Nm1, Nm2, to which applies:
Nm1xe2x89xa71.05NLS
Nm2xe2x89xa60.95NLS.
The first material m1 of the mixed material is preferably also a material used as the material with the higher refractive index, thus an oxide or oxynitride, therein preferably at least one of the above listed materials TiO2, Ta2O5, Nb2O5, HfO2, ZrO2, SiOxNy, especially preferred TiO2 and/or Ta2O5.
As the second material m2 of the mixed material is preferably used SiO2 and/or Al2O3 and/or SiOxNy and/or Y2O3. By selecting the mixing ratio m1/m2 of the mixed material, the material with the lower refractive index, the desired refractive index NLS is realized. As the material with the lower refractive index, a mixed material is preferably used comprising SiO2 and TiO2 with an SiO2 fraction ASiO2=(60xc2x15)% and a TiO2 fraction ATiO2 of (100%xe2x80x94ASiO2). Y2O3, having a refractive index of 1.8xc2x12%, can therein also be used as the material with the lower refractive index.
The refractive index NK of the material of the at least one base body is preferably selected as follows:
1.52xe2x89xa7NKxe2x89xa71.
The value 1.52 corresponds therein to the refractive index of BK7 glass which is commonly used in particular for X-cubes. The stated body can also be fabricated of quartz glass with
NK=1.45xc2x12%.
It is preferred if on the light splitter according to the invention both coating systems are implemented like said at least one. Furthermore, according to the invention and especially preferred, a light splitter implemented as an X-cube of the above described type is proposed, in which the coating systems are embedded substantially along diagonal planes of a base body cuboid which is preferably square in a cutting plane perpendicular to the common line of intersection of the coating systems.
To summarize: thus a formulation according to the invention of the solution of the above task is the specific selection of materials of the described coating systems.
A second solution of the above task is attained through a spectrally selective light splitter. In such a light splitter, which splits white light into red, green and blue light, respectively recombines white light from red, green and blue light, with two separate coating systems, each reflecting and transmitting color selectively, which are applied, or are embedded, on or in at least one transparent base body and on which the body defines emergent faces for the red, green and, respectively, blue light, at least on of the emergent faces with respect to the direction of the light emerging from it and reflected on at least one of the coating system is tilted such that the face normal of the emergent face forms with the stated direction, and acute angle xcfx86 which differs from 0xc2x0. Due to fabrication tolerances on the light splitter, angles between light emergent direction and the face normal differing from 0xc2x0 may well occur in this case, but, on the one hand, such tolerance-dependent angle deviations are not reproducible and, on the other hand, the tolerance-dependent angle deviations due to production are prior known. The zero deviation realized according to the invention, in any event, is greater than the stated prior known tolerance deviation.
Preferably, and as yet to be explained, the addressed maximum angle deviations are 5xc2x0.
It is now possible to apply directly onto the emergent faces on such a light splitter light valves, operating in reflection with which, with respect to the coating systems, for example according to FIG. 2, light in S-polarization, incident on a considered coating system and to be spectrally divided, is reflected on the coating system at a different angle than the P-polarized light to be recombined of the same spectrum reflected by light valve RLV.
However, the possibility also exists of realizing the reflection angle shift on one and/or the other coating system without corresponding tilting of the emergent faces on the light splitter itself, i.e. without changing the light splitter itself by mounting the light valves at a corresponding relative tilt.
A corresponding optical transmitter configuration with a light splitter, with an incident and emergent face for white light and emergent faces each for red, blue and green light, comprises reflectors, operationally connected with the emergent faces of the stated light splitter, which reflectors change the polarization of light and on which the light emergent at the emergent faces is reflected at an angle deviating from 0xc2x0 by more than is given through the fabrication tolerances of the total configuration. Such an optical transmitter configuration can now be realized through the direct application of light valves, operating in reflection, onto the tilted emergent faces of a light splitter or the reflector tilt can be realized through mechanical mounting measures, apart from the light splitter implementation proper.
The light splitter according to the invention considered by itself comprises preferably in combination the measures with respect to layer material as well as also the measures of realizing on the coating systems differing reflection angles specific to the direction of the incidence of rays, as is specified by the light splitter. Furthermore, an optical transmitter, in principle with tilted reflectors, preferably comprises also the solution characteristics with respect to specific layer materials on the light splitter.
The light splitter according to the invention considered by itself comprises preferably in combination the measures with respect to layer material as well as also the measures of realizing on the coating systems differing reflection angles specific to the direction of the incidence of rays, as is specified by the light slitter according to claim 10. Furthermore, an optical transmitter, in principle with tilted reflectors according to claim 12, preferably comprises also the solution characteristics with respect to specific layer materials on the light splitter as specified according to the wording of claim 13.
Each of the principles according to the invention, namely the material selection according to the invention of one, preferably of both of the coating systems and/or reflection specific as to direction of incidence on at least one, preferably on both coating systems, now permits working with reflecting light values in particular also in the case of projector configurations.
An optical transmitter configuration, on which the stated tilt is realized and/or which comprises a light splitter with the material selection according to the invention of the coating systems, preferably comprises further a polarization beam splitter at the input side and/or an HMI lamp as an illumination source.
Said HMI lamp has in its light spectrum low energy values where the range of spectral shift range also is located of the reflection behavior with respect to S- and P-polarized light, i.e. corresponding to the spectral ranges xcex94R and xcex94B according to FIGS. 3 and 7.